1. Field of the Invention
The present invention relates to extrusion apparatus for producing elongated, perforated products wherein the perforations are formed in co-extending relation in predetermined patterns. More particularly, this invention relates to extrusion product flow control or metering devices for use with the aforementioned apparatuses wherein the pattern of the perforations can be varied without extensive modification being required to the apparatus or its components.
2. Description of the Prior Art
It is known in the art to form elongated articles by extrusion with one or more co-extensive perforations internally formed therein. For example, gun propellants, usually formed by extrusion from curable, plastic compositions of a solid explosive finely dispersed in a binder material, also usually contain several internal, co-extensive perforations arranged in a predetermined pattern. These compositions are highly viscous materials formed by means of an extrusion apparatus through an extrusion die into elongated, continuous strands which are thereafter collected in a container or laid onto a rotating aluminum table, or "skid", which is coated with a suitable release agent, and thereafter cured. After curing the strands are cut to pellet sized lengths in a cutting machine for use as gun propellant in artillery shells and the like.
The extrusion die of the apparatus normally is contained in a die body of the machine having a central bore as an extrusion passage. A counterbore or entrance chamber is usually provided in the body communicating with the bore and a cover or support plate is installed as a closure for the chamber. In the usual instance, one or more perforations or extrusion holes are formed in the cover through which the extrusion material flows before entering and filling the chamber and the central extrusion passage, or bore in the die body. The size and shape of the bore, in general, corresponds to and thereby determines the size and configuration of the extrusion product.
To form the co-extending perforations, the cover or support plate carries a plurality of pins fixed at one end and supported in the cover in cantilever fashion, the free ends of the pins extending into and through the chamber and into the extrusion hole or bore in the die body. The pins are arranged as mandrels in a "bolt circle" pattern and the extrusion product flows around and between them forming the perforations in a pattern determined by the number and placement of the pins in the bolt circle. The number of pins used corresponds to the number of perforations desired. The presence of perforations in a propellant product is desired to provide increased burning surface and to insure the propellant is fully consumed and burned when the gun is fired. In general, the preferred perforation pattern is one with seven perforations arranged in a bolt circle configuration of six perforations surrounding the seventh in the center.
Heretofore it has always been difficult to extrude elongated products using the aforementioned pins as mandrels for forming the perforations owing to the fact that their extreme lengths compared to their small diameters coupled with their cantilever attachments to the cover, subjects their free ends to undesirable, random displacement or positioning during extrusion. This free end movement in turn tends to alter the perforation pattern in undesirable and unpredictable ways. Thus, depending on how much extrusion product flows into and fills the space between the pins and whether the area outside the pins is filled sooner or later, which cannot be predicted with accuracy, the pins will be forced outwardly in a diverging pattern, or inwardly in a converging pattern.
In the prior art, various devices have been used to divert some of the extrusion product away from the periphery of the die bore and delay its arrival thereat until a significant part of the pin bolt circle interior area has been filled with extrusion product, after which the diverted product is allowed to flow into and comingle with the center flowing product portion. In effect this tends to cause the center pin extrusion product to act as an extrusion fixture to stabilize the pins and maintain their ends parallel until the diverted product arrives and fills the surrounding area. The device is usually installed in the chamber or counterbore and has a top surface close to the pin attachment point to reduce pin length exposed to bending moment introduced by the product material flow against the sides of the pins. The surface provides a means for diverting some of the flow away from the pins to the outer region of the chamber, while the remaining product flows centrally to the pin interior region of the chamber through a central hole in the device. The material flows downwardly through the hole and adopts the shape of a shaft with the pins embedded in its outer surface and, as it exits the hole at the entrance to the die bore, it is met by the diverted material flowing in from the outer chamber region through passages also provided in the device, and the combined product continues through the bore exiting downstream as a continuous, elongated perforated extrusion product.
While use of the above mentioned distribution device has been effective to reduce pin movement and thus maintain a relatively uniform perforation pattern throughout the length of the extrusion product, it is often required to produce varying perforation patterns in such products. When so required, however, the variation should be predictable as based on a predetermined plan without requiring changes of equipment components or modification to any of the existing component parts such as, for example the cover or pin support plate, the pins and arrangement thereof, or the geometry of the die body or basic die assembly itself. Heretofore to accomplish this it has been necessary to provide a new die or die body, a new support plate or cover, to change the mandrels or perforation pins or to arrange them in new patterns, or to modify some or all of the other parts of the extrusion apparatus. It has long been known to obtain a varying pattern of perforations in an elongated extrusion product requires controlling the extrusion product flow rate by altering the direction or delaying the flow thereof within the extrusion die body in more or less specific ways. That is to say, the flow of extrusion material, particularly in the upper die body in the initial stages of the extrusion process, must be adjusted, as by metering, or otherwise altered in such a way that pin placement either remains undisturbed, i.e. random movement of the pins is prevented, or changed purposefully to provide the required variation in the perforation pattern. The present invention provides a means for accomplishing these ends by metering the flow of product in the die body so that a portion is delayed by redirection or rerouting over a relatively more circuitous path to the outer area or region of the die body chamber until the center area between the pins is filled by the remaining or undiverted product portion. In the present invention, extrusion product flow adjustment is accomplished and the perforation pattern varied accordingly without changes of or modifications to any of the other more difficult to supply or modify parts of the apparatus being required. In addition, no pin changes are required and the same support plates can be used. The only change required is the addition of an inexpensive, easy to fabricate adjustment or metering device in the form of a plate or disc positioned in the chamber between the aforesaid distribution device and the outlet side of the cover or pin support plate.